The present invention relates to an active matrix liquid crystal display which controls the orientation of liquid crystal by applying a lateral electric field in a direction along the surface of a substrate and particularly, relates to structure which enables the enhancement of the numerical aperture.
Recently, there is a problem in a twisted nematic (TN) mode liquid crystal display that dependency upon the angle of visibility is high because vertical visibility is poor though horizontal visibility is relatively good. Therefore, the applicants of the present invention formerly developed a liquid crystal display provided with structure which enabled the solution of such a problem.
According to technique for solving the problem, an electrode for driving liquid crystal is not respectively provided to the upper and lower substrates which holds a liquid crystal layer between them but two types of linear electrodes 12 and 13 with different poles are provided only to the lower substrate 11 with the above electrodes mutually separated as shown in FIG. 10, no electrode is provided to the upper substrate 10 shown in FIG. 10 and liquid crystal molecules 36 can be oriented in the direction (the direction of the surface of the substrate) of a lateral electric field generated between both linear electrodes 12 and 13 by applying voltage.
That is, as shown in FIG. 9, a comb-type electrode 16 is constituted by connecting each linear electrode 12 to a base line part 14, a comb-type electrode 17 is constituted by connecting each linear electrode 13 to a base line part 15, both comb-type electrodes 16 and 17 are combined, a switching element 19 is connected to the base line part 14 and a power source 18 is connected to the base line part 15.
As shown in FIG. 11A, a polarizing plate provided with the polarization direction of a direction .beta. shown in FIG. 11A is laminated on the upper substrate 10 by forming an orientation film on the liquid crystal side of the upper substrate 10 and applying orientation processing to the orientation film so that liquid crystal molecules 36 are arranged in the direction of .beta., and a polarizing plate provided with the polarization direction of a direction .alpha. is laminated on the lower substrate 11 by forming an orientation film on the liquid crystal side of the lower substrate 11 and applying orientation processing to the orientation film so that liquid crystal molecules 36 are arranged in the direction of .gamma. parallel to the above direction .beta..
According to the above constitution, liquid crystal molecules 36 are homogeneously oriented in the same direction in the orientation direction of the orientation films as shown in FIGS. 11A and 11B in a state in which no voltage is applied between the linear electrodes 12 and 13. Back light which passes the lower substrate 11 in this state is polarized in the direction of .alpha. by the polarizing plate, as the back light is transmitted in the layer of liquid crystal molecules 36 as it is and reaches the polarizing plate with a different polarization direction .beta. of the upper substrate 10, it is interrupted by the polarizing plate and as no back light passes a liquid crystal display, the liquid crystal display is in a dark state.
Next, when voltage is applied between the linear electrodes 12 and 13, liquid crystal molecules 36 are twistedly oriented as shown in FIGS. 12A and 12B. In this state, the polarization direction of light transmitted in the lower substrate 11 and polarized in the direction of a is converted by the twisted liquid crystal molecules 36, the light can pass through the upper substrate 10 provided with the polarizing plate with the polarization direction .beta. different from the direction .alpha. and the liquid crystal display is in a light state.
FIGS. 13A, 13B and 13C show the structure of a liquid crystal display provided with the linear electrodes 12 and 13 provided with the above structure in case the structure of the liquid crystal display is applied to an actual active matrix liquid crystal driving circuit.
FIG. 13A shows the planar structure of each electrode, FIGS. 13B and 13C show the sectional structure, gate wirings 21 and source wirings 22 arranged in a matrix on a substrate 20 are formed on/over the transparent substrate 20 via an insulating layer 24 between them and a part equivalent to each rectangular area surrounded by the gate wirings 21 and the source wirings 22 is a pixel area. Further, a gate electrode 21a composed of a part of the gate wiring 21 is formed at the corner of each pixel area, a semiconductor layer 26 is formed on the insulating layer 24 on the gate electrode 21a and a thin film transistor T is constituted by a source electrode 27 and a drain electrode 28 on both sides of the semiconductor layer 26.
A common wiring 30 is provided next to the gate wiring 21 on the substrate 20 so that the common wiring passes each pixel area and common electrodes 31 are provided in a part equivalent to each pixel area of each common wiring 30 so that the common electrode is adjacent to the source wiring 22 located on both sides of each picture element and the ends of the common electrodes 31 are connected by a common electrode connection 32 provided along the gate wiring 21 on the end side of the common electrode 31.
Further, the drain electrode 28 provided to the thin film transistor T is connected to a capacity generating electrode 33 extended on the upper side of the common electrode connection 32, the capacity generating electrode 33 is connected to a pixel electrode 34 provided in the middle of the common electrodes 31, the pixel electrode 34 is extended to the side of the common wiring 30 and is connected to a capacity generating electrode 35 formed on the common wiring 30.
In the structure shown in FIGS. 13A to 13C, as a lateral electric field can be let to act so that a line of electric force is formed in a direction shown by a dotted arrow a in FIG. 13C, liquid crystal molecules can be oriented according to the lateral electric field. Therefore, a light state and a dark state can be switched by controlling the orientation of liquid crystal as in the case described above, referring to FIGS. 11 and 12.
However, the liquid crystal display provided with the above structure has a problem that the numerical aperture is often small though the display has an advantage that the angle of visibility is large.
That is, in the structure shown in FIGS. 13A to 13C, the orientation of liquid crystal molecules is controlled by a lateral electric field generated between the pixel electrode 34 and each common electrode 31. However, since the gate wiring 21 and the common wiring 30 are both directly formed on the substrate 20 as shown in FIGS. 13B and 13C and located in the same plane, fixed space d.sub.1 is required to be left between the gate wiring 21 and the common wiring 30 to prevent a short circuit in wiring as shown in FIGS. 13A and 13B. Because the space d.sub.1 may be probably a defective part in which the leakage of light and others are caused, the space is required to be covered with a black matrix and others and therefore, a numerical aperture as a liquid crystal display cannot be enhanced. As the orientation of an electric field applied to a liquid crystal molecule is different from that of a lateral electric field in each area over the common electrodes 31, the orientation of liquid crystal molecules 36 is different from that of an area between the pixel electrode 34 and the common electrode 31 as shown in FIG. 10.
Therefore, heretofore, as an area over the common electrode 31 may cause a problem such as the leakage of light, structure that the area is covered with a black matrix is adopted and further, structure that the periphery of a part covered with a black matrix is located slightly inside the inner edge of each common electrode 31 is adopted, therefore, an area covered with a black matrix is widened and there is a problem that a numerical aperture as a liquid crystal display cannot be enhanced.